Field of the Invention
The present invention relates to communication cables, particularly optical cables that contain bundles of optical fibers.
Discussion of the Known Art
U.S. Pat. No. 6,807,347 (Oct. 19, 2004) discloses a high density fiber optic cable in which groups of fibers are each harnessed or bound by a binder thread to form a number of fiber bundles. All relevant portions of the '347 patent are incorporated by reference.
When installing a cable that contains bundles of optical fibers, and each bundle is tied with a binder thread as in the referenced '347 patent, the following problems arise when the cable jacket is stripped or removed from one end of the cable in order to terminate or splice the exposed fibers.
1. In the absence of the cable jacket that surrounds the fiber bundles, the binder threads loosen or separate from the associated fiber bundles and fail to restrain the fibers of each bundle from moving about freely. As a result, fibers originating from different bundles will often tend to align and commingle with one another, making it difficult for an installer to determine to which bundle a given fiber may belong. The installer must therefore use special care and take additional time when stripping the cable jacket to access the bundled fibers.
2. To preserve the integrity of each of the fiber bundles, the installer often strips off a relatively long piece of the cable jacket so that the binder threads will remain physically tied around at least some portions of the exposed bundles. All the long pieces of the stripped cable jacketing create excess scrap, and scrap reduction is essential if the costs associated with cable installations are to be controlled.
FIG. 1 illustrates a fiber bundle 10 including a group of four fibers 12 encircled by a binder thread 14 as in the prior art. FIG. 2 is a cross sectional view or profile of the fiber bundle 10 in FIG. 1. Each fiber has a central core 16, a cladding layer 18 that surrounds the core 16, and an outer coating layer 20 typically formed of UV curable acrylate that surrounds the cladding layer 18. The fibers 12 can have any one of a number of different outer diameters, for example and without limitation, 250 μm or 200 μm which are common diameters.
FIG. 3 is a cross sectional view through a fiber optic cable 30 having an outer jacket 32 and which contains two fiber bundles 34, 36. Each bundle includes, for example, 12 fibers that are encircled by a corresponding binder thread 38, 40 as in the prior cables. A layer of strength elements 42 (e.g., aramid yarn) is interposed between the fiber bundles 34, 36 and the outer jacket 32 of the cable 30.
FIG. 4 shows one end of the cable 30 in FIG. 3, after a length of the outer jacket 32 and the strength elements 42 are removed to expose the fiber bundles 34, 36. The binder threads 38, 40 that encircle and restrain the bundled fibers beneath the jacket 32, become loose and easily separate from the associated bundles. As a result, the threads 38, 40 no longer adequately restrain the fibers from movement. Fibers from both of the fiber bundles 34, 36 will then tend to align and commingle with one another as shown in FIG. 4, where an unrestrained fiber 34a from bundle 34 becomes mixed among the fibers from bundle 36, and an unrestrained fiber 36a from bundle 36 becomes mixed among the fibers from bundle 34.
Accordingly, there is a need for a fiber optic cable of the kind that contains a number of fiber bundles, wherein each bundle is tied by a binder thread that remains in place and continues to restrain movement of the bundled fibers after the cable jacket and other cable elements in proximity to the bundles are removed.